Relativity (the fact that the laws of motion are the same in all inertial frames of reference) is itself a crucial part of Newtonian mechanics, except that Eins

Relativity (the fact that the laws of motion are the same in all inertial frames of reference) is itself a crucial part of Newtonian mechanics, except that Einstein's theories of relativity were such a big deal that we now tend to reserve the word "relativity" for his stuff, and call the old thing "Galilean invariance" or similar.